This invention relates to a layer system having a controllable heat emission.
The heat balance of a spacecraft is essentially determined by the interplay of three effects: 1) radiation from the outside, predominantly from the sun and the earth; 2) internal heat sources, such as electric equipment; and 3) heat radiation into space.
Layer systems with a controllable heat emission, in the following also called electroemissive elements, permit the control of radiation-caused heat flows with a low expenditure of energy and without any mechanical functions. A known state of the art electroemissive element shown in FIG. 1 consists of a multilayer system on an infrared-transparent carrier substrate 1. The layer system 2, 3, 4, 5, 6 comprises at least one electroemissive layer 2, an ion-permeable and infrared-reflecting electrode 3, an infrared-absorbing solid electrolyte 4, an ion storage layer 5 and a back electrode 6.
The function of the component is based on the electrically controlled change of the infrared (IR)-transmission of the electroemissive layer in front of an "infrared mirror." The surface of the component is the side of the substrate which is illustrated to be uncoated in FIG. 1. From there, heat radiation passes through the IR-transparent substrate largely in an unhindered manner. When the electroemissive layer 2 is transmissive, this heat radiation is reflected by the electrode situated behind it. In this case, the element itself can emit little heat. When, in contrast, the electroemissive layer is switched to be absorptive, a reflection of the heat radiation, which impinges from the outside, on the electrode 3 is no longer possible. The electroemissive layer 2 will now emit the heat radiation corresponding to its temperature. Only by means of the arrangement of the electrode 3 between the electroemissive coating 2 and the IR-absorbing solid electrolyte 4, is it possible to make the IR-effect visible.
The function of the electroemissive (EE)-element and the object of the electrode 3 will be explained in detail by way of an example, in which tungsten oxide is used as the electroemissive layer 2.
The infrared-optical switching operation is caused in the electroemissive layer through use of an electrochemical process. While receiving electrons and a simultaneous storing of ions from an electrolyte, a negative potential on the electrode 3 results in the reduction of the electroemissive layer 2. Inversely, while electrons and ions are emitted, an oxidation process takes place on the back electrode 6 or the ion storage layer 5. The electronically insulating solid electrolyte 4 takes over the necessary ion transport between the ion storage layer 5, through openings of the electrode 3, to the electroemissive layer 2. The electric field between the two electrodes 3 and 6 is the driving force for the operation. The reduced electroemissive layer 2 is IR-absorptive and therefore high-emitting.
When a positive potential is applied to the electrode 3, the reverse process will take place. The electroemissive layer 2 is oxidized while emitting electrons and ions and the storage layer is reduced at the same time. The electroemissive layer 2 becomes transmissive.
From German Patent documents DE 36 43 691 and DE 36 43 692, electro-optical components are known. Through the application of an electric voltage, these components have a degree of heat emission which can be switched to a high-emitting condition or, by a pole inversion, can be switched into a low-emitting condition. These components are predominantly suitable for military applications for camouflaging and for civil applications, for example, as a variable thermal control layer for spacecraft and satellites. These systems have the disadvantage that the ion transparency of the electrode 3 is essentially dissolved by the diffusion of a soluble species, such as protons (H.sup.+), in precious metal layers (palladium, gold) and/or by the microporosity of thin gold layers on a rough base (in the present case, the electroemissive layer 2). The driving force of the diffusion by the electrode 3 is the concentration gradient, which occurs according to the poling, between the electroemissive layer 2 and the solid electrolyte 4. The concentration gradient is created by the enhancement or depletion of ions of the solid electrolyte when the electric field is applied in the close surroundings of the electrode 3. The voltage of clearly above 1 volt required for the control, in the case of aqueous electrolytes (such as aqueous polymer solid electrolytes with lithium conducting salt), results in undesirable secondary reactions on the electrode 3 (for example, hydrogen development), whereby the useful life of the component is limited to less than 100 switching cycles. Lower voltages in the range of from 0.7 to 1 volt permit only a low emission stroke of below 25% (.DELTA..epsilon..sub.type =0.4 . . . 0.65), and switching times of approximately 15 minutes at 22.degree. C.
There is therefore needed an improved arrangement and structuring of the known layer systems with a controllable heat emission which meets all of the following requirements:
1) no impairment of the function of the electrode 3; that is, high electronic conductivity over the entire surface; PA1 2) high permeability for ions; that is, no significant resistance against ion flow; PA1 3) optimal contact between solid electrolyte 4 and electroemissive layer 2; and PA1 4) optimal reflectivity of the electrode 3 in the wavelength range&gt;2.5 .mu.m in the direction of the electroemissive layer 2.
These needs are met according to the present invention by providing a layer system with a controllable heat emission. The layer system includes an IR-transparent carrier substrate, a layer with a controllable IR-transmission, an ion-permeable, IR-reflecting electrode, a solid electrolyte, an ion storage layer, and a back electrode. The ion-permeable, IR-reflecting electrode is structured in such a manner that it is provided with non-cohesive openings which are distributed homogeneously over the electrode surface and whose maximal flat dimension is smaller than 10 .mu.m.
The solution according to the present invention relates particularly to the ion-permeable, IR-reflecting electrode. According to the invention, this electrode is structured in such a manner that it is provided with non-cohesive openings which are distributed homogeneously over the electrode surface and whose maximal plane dimension is smaller than 10 .mu.m. The structuring takes place in such a manner that an ion transport is permitted between the electroemissive layer and the solid electrolyte and, at the same time, a high reflectivity is achieved for the radiation of the wavelengths&gt;2.5 .mu.m.
Advantageously, the structuring of the electrode is carried out according to microstructuring methods which are known per se and which are used, for example, for the structuring of semiconductor components.
With respect to the dimensions of the openings in the ion-permeable, IR-reflecting electrode, the following interrelationships apply: Advantageously, the dimensions of the openings should be smaller than the wavelengths of the radiation to be reflected. Since, in practical applications, naturally, there is never a sharply defined wavelength range of the incident radiation, it is sufficient in practice for the dimensions of the openings to be smaller than those wavelengths which together make up a significant portion of the overall intensity of the incident IR-radiation to be reflected.
As a rule, the incident wavelength distribution corresponds to the radiation of a black body. In the case of a suitable dimensioning of the openings in the range of less than or equal to 10 .mu.m, it is possible to achieve a sufficient reflectivity of the electrode for IR-radiation over a large range of application temperatures.
The lower limit of the diameters for the openings is essentially determined by the microstructuring methods possible today.
Suitable materials for the ion-permeable, infrared-reflecting electrode are all conductive materials, such as metals and semiconductors with a high reflectivity for infrared radiation (particularly&gt;2.5 .mu.m). Examples in this respect are particularly metals, such as Au, Ag, Pt, Pd, Al, Cu, Fe, Pb, Ni, Cr, and mixtures thereof, and semiconductors, such as appropriately doped Si, Ge as well as indium oxide, tin oxide, zinc oxide and mixtures thereof. In a particularly advantageous embodiment, the electrode consists of a triple layer of Ti/Au/Ti. The two Ti-layers serve as an adhesive base for the respective adjoining layers (electroemissive layer, solid electrolyte).
The layer thickness of the ion-permeable, infrared-reflecting electrode is preferably in the range of from approximately 0.1 .mu.m to 1 .mu.m. Such layers have the advantage of increased mechanical stability and increased electronic conductivity. In contrast, porous electrodes, as known, for example, according to German Patent document DE 36 43 691 C1, are much thinner (approximately 0.01 .mu.m). There, the transport of the ions through the electrode takes place mainly through the pores of the electrode. For ensuring a sufficient porosity, these electrodes must be correspondingly thin, with the resulting negative consequences for the conductivity and the mechanical stability.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.